1. Field of the Invention
The present invention relates to a hybrid construction machine which uses both engine power and electric power.
2. Description of the Related Art
Known techniques relating to the present invention will be described. Here, an excavator will be described for illustrative purposes.
As illustrated in FIG. 8, an excavator includes a crawler-type lower traveling structure 1 and an upper rotating structure 2 rotatably mounted on the lower traveling structure 1. A working attachment 3 is mounted on the upper rotating structure 2. The working attachment 3 includes a boom 4 capable of moving up and down, an arm 5 attached to an end of the boom 4, a bucket 6 attached to an end of the arm 5, and cylinders 7, 8, and 9 which are hydraulic actuators for driving the boom 4, the arm 5, and the bucket 6, respectively.
If the excavator is a hydraulic excavator, a hydraulic motor is used as a rotation actuator for rotating the upper rotating structure 2. On the other hand, if the excavator is a hybrid excavator, a rotation motor, which is an electric actuator, is used as the rotation actuator.
A hybrid excavator of a so-called parallel drive type is known as disclosed in Japanese Unexamined Patent Application Publication No. 2001-12274.
In this parallel-drive-type hybrid excavator, a hydraulic pump and a generator motor performing both a generator function and a motor function are connected in parallel to an engine, which serves as a common power source. The hydraulic pump drives a hydraulic actuator, while an electric storage device drives a rotation motor. The electric storage device is charged by the generator motor. Timely, discharge power of the electric storage device allows the generator motor to assist the engine by performing the motor function.
Therefore, in the parallel-drive-type hybrid excavator, the maximum power of the entire system is determined by the sum of engine power and electric storage device power (i.e., stored electric power).
Since a hybrid excavator is designed to use a small engine, the maximum power of the engine is lower than the maximum output of the hydraulic pump. This deficiency in engine power is made up by the electric storage device via the generator motor.
A problem to be solved by the hybrid excavator is how to provide substantially the same performance and operability as those of a hydraulic excavator with equivalent operating capabilities. Therefore, the maximum system power of the hybrid excavator is determined based on the maximum power (i.e., maximum engine power) of the hydraulic excavator with equivalent operating capabilities and is typically set to be higher than the maximum power of the hydraulic excavator.
However, if the maximum system power of the hybrid excavator is set to be higher than the maximum power of the equivalent hydraulic excavator, the power of the hybrid excavator may exceed an appropriate level depending on a load or the like. Moreover, when an operator operates the hybrid excavator in the same way as in the case of the hydraulic excavator, the resulting movement of an actuator of the hybrid excavator may differ from that of an actuator of the hydraulic excavator. This can be uncomfortable for an operator who is familiar with the operation of the hydraulic excavator. In this respect, the operability of the hybrid excavator becomes worse.
At the same time, even if the maximum system power of the hybrid excavator is set to be higher than the maximum power of the equivalent hydraulic excavator, the maximum system power may be insufficient in the case that a load exceeding the set maximum system power is applied.
In such a case, the hydraulic excavator can cope with power deficiency by performing control to reduce the pump output. However, in the hybrid excavator, which includes the electric storage device as a drive source in addition to the engine, the electric storage device may cause an abnormal voltage drop or over-discharge thereof in an attempt to respond to the applied load. This may shorten the life of the electric storage device or cause damage to the electric storage device. Moreover, if the amount of assist supplied from the electric storage device is insufficient, an excessive load applied to the engine may cause engine stalling. This problem becomes apparent when the hydraulic actuator and the electric actuator are simultaneously operated, because the total loads increase.
The above-described patent document discloses a technique in which, in the hybrid construction machine of a parallel drive type, in the case that the power consumption of the hydraulic pump exceeds engine power, the generator motor performs the motor function while the pump output is reduced. This assists the engine and thus can make the engine revolution speed constant.
However, this technique achieves only a function of efficiently using a small engine, that is, the originally intended function of the hybrid construction machine. In this technique, the pump output is reduced on the basis of only the engine power, not on the basis of the maximum power of the entire system determined by the sum of engine power and electric storage device power. Since this causes unnecessary degradation in operating performance of the hybrid construction machine, its primary object of providing substantially the same performance as that of the equivalent hydraulic construction machine cannot be achieved.